Department für Chemie, Universität zu Köln , 50939 Köln, Germany.
J Org Chem. 2017 Aug 18;82(16):8476-8488. doi: 10.1021/acs.joc.7b01223. Epub 2017 Jul 31.
Second-order rate constants (k) of the reactions of various barbiturate anions such as the parent barbiturate, 1,3-dimethylbarbiturate, 2-thiobarbiturate, and 1,3-diethyl-2-thiobarbiturate with diarylcarbenium ions and Michael acceptors have been determined in dimethyl sulfoxide solution at 20 °C. The reactivity parameters N and s of the barbiturate anions were derived from the linear plots of log k versus the electrophilicity parameters E of these reference electrophiles, according to the linear-free-energy relationship log k (20 °C) = s (E + N). Several reactions of these nucleophiles with benzylidenemalononitriles and quinone methides proceeded with reversible formation of the new C-C-bond followed by rate-determining proton shift. No evidence for initial attack of the electrophiles at the enolate oxygens of these nucleophiles was found by the kinetic measurements, in line with quantum chemical DFT calculations, which showed that in all cases C-attack is kinetically and thermodynamically preferred over O-attack. The nucleophilic reactivities of barbiturate anions were compared with those of structurally related carbanions, e.g., Meldrum's acid and dimedone anions.
在 20°C 的二甲亚砜溶液中，测定了各种巴比妥酸盐阴离子（如母体巴比妥酸盐、1,3-二甲基巴比妥酸盐、2-硫代巴比妥酸盐和 1,3-二乙基-2-硫代巴比妥酸盐）与二芳基碳鎓离子和迈克尔受体的反应的二级速率常数（k）。根据线性自由能关系 log k（20°C）= s（E + N），根据这些参考亲电试剂的电性质参数 E，从 log k 对这些亲核试剂与亚苄基丙二腈和醌甲烷化物的几种反应的线性图中推导出了巴比妥酸盐阴离子的反应参数 N 和 s。动力学测量没有发现这些亲核试剂的亲电试剂初始攻击它们的烯醇化物氧，这与量子化学 DFT 计算一致，表明在所有情况下，C-攻击在动力学和热力学上都优先于 O-攻击。与结构上相关的碳负离子（例如，Meldrum 酸和二酮负离子）相比，比较了巴比妥酸盐阴离子的亲核反应性。
